The present invention relates to signal error reduction in receiving apparatus, for example signal error reduction in a receiver operable as part of a GSM (Global System for Mobile Communications) digital radio communication system.
In a digital communication system, a transmission signal is produced by modulating a carrier signal with the digital data to be transmitted. The digital data is commonly transmitted in bursts, each burst consisting of a predetermined number of data bits. Various different types of modulation may be used, of which amplitude, frequency and phase modulation are the most common.
In receiving apparatus of such a digital communication system, the received signal must be demodulated to derive therefrom the information content (transmitted digital data). In one demodulation technique, referred to as direct down conversion, in the receiving apparatus a complex (quadrature pair) local oscillator operating at the carrier frequency is used to mix down the received signal to produce inphase (I) and quadrature (0) baseband signals, also referred to sometimes as zero IF signals. Alternatively, such I and Q baseband signals may be produced after processing in an intermediate frequency (IF) stage.
These inphase and quadrature baseband signals can then be processed, for example on a burst-by-burst basis, to derive therefrom the information content of the received signal.. It is convenient to carry out such processing as far as possible in the digital domain, and for this reason the inphase and quadrature baseband signals in all of the bit periods of a burst may be converted into a set of digital I and Q signal-value pairs, each pair comprising an I-value and a corresponding Q-value representing respectively the inphase and quadrature baseband signals in a particular bit period.
The digital I- and Q-values of such signal-value pairs can be used to analyze the received signal for the purposes of extracting the information content therefrom. In particular, for a phase- or frequency-modulated constant-amplitude received signal the signal-value pairs would, if plotted on a complex signal space diagram (I/Q diagram), lie substantially on a common circle, the angular positions of the plotted signal-value pairs with respect to the circle centre being then used by a digital equalizer to derive raw data from the signal-value pairs.
However, performance of a such a digital equalizer is found to be degraded seriously by DC errors in the I- and Q-values of the signal-value pairs. Such DC errors may arise due to mismatches between the down conversion mixers used to produce the analog baseband inphase and quadrature signals, and due to DC offsets in the analog signal processing circuitry, used to process those baseband signals prior to conversion into digital signal-value pairs.
These DC errors may vary with time and temperature, and may also be larger than the amplitude of the wanted signal in the case of weak signals.
It is possible to remove DC errors that are larger than the signal amplitude by averaging the I-values of all the signal-value pairs over a burst and then subtracting the average I-value from the I-value of each pair, and by averaging the Q-values of all the signal-value pairs over a burst and subtracting the average Q-value from the Q-value of each pair. These DC cancellation operations must be carried out separately for I and Q, because the DC errors in I- and Q-values will be different and unrelated.
These DC cancellation operations (average subtraction) are not however, sufficient alone to deal with the DC error problem. Subtracting the average signal level actually introduces a new DC error of its own because it removes all DC from the received signal, whereas in practice the received signal itself will almost certainly have a DC content which should not be removed. The DC content is not constant, and varies from one burst to the next according to the digital data included in the burst. There is therefore, a need for a way of restoring this variable DC content to the received signal following an initial DC cancellation operation and such a DC restoration operation must be performed in a short time period consistent with the burst repetition rate of the communication system.